The increasing demand for high data rates in cellular networks requires new approaches to meet this expectation. A challenging question for operators is how to evolve their existing cellular networks so as to meet the requirement for higher data rates. In this respect, a number of approaches are possible: i) increase the density of their existing base stations, ii) increase the cooperation between base stations, or iii) deploy smaller base stations (low power nodes, or LPNs) or relay nodes (RNs) in areas where high data rates are needed within a base stations grid.
The option of deploying smaller basestations is in general referred to in the related literature as a “Heterogeneous Network”, or “Heterogeneous Deployment” and the layer consisting of smaller base stations is termed a “micro”, or “pico” layer. The original larger base stations are then referred to in this context as “macro” base stations.
Building a denser macro base station grid, while simultaneously enhancing the cooperation between macro base stations (hence either using options i) or ii) above) is a solution that meets the requirement for higher data rates; however such an approach may not necessarily be a cost-efficient option, due to the costs and delays associated with the installation of macro base stations, especially in urban areas where these costs may be significant.
FIG. 1 shows the basic principle of heterogeneous deployments. Large macro cells 10, which are geographic areas nominally served by a base station, are generally able to provide coverage to a larger service area. However, the addition of smaller micro/pico cells 11 can improve network capacity in certain regions of those macro cells. micro/pico cells are essentially subcells of a macro cell, and are served by low power, short range nodes, such as micro/pico base stations, using frequencies allocated by the macro base station. Allocation of resources between the macro and micro/pico cells can be semi-static, dynamic or shared across the macro-micro/pico layers.
A relay node (RN) is a particular type of low power node that can be provided for enhancing the coverage and capacity of a macro base station. A relay node typically implements a reduced protocol stack as compared to a typical micro or picocell base station. In its simplest form, a relay node is simply a repeater that receives and amplifies a signal from a macro base station. More complex relay nodes may implement higher protocol levels, such as media access control (MAC) layer functionality, up to layers such as mobility management, session set-up and handover.
Referring to FIG. 2, a relay node 30 provides an intermediate node between a user equipment unit (UE) 40 and an eNodeB 20, referred to in this context as a “donor”, “serving” or “anchor” eNodeB, as it is providing resources to the relay node 30. Communications between the relay node 30 and the UE 40 is performed using the Uu interface, which is the same interface that the UE 40 normally uses to communicate directly to the eNodeB 20; from the standpoint of the UE 40, there is no difference in the protocol used when communicating with a relay node.
Communications between the relay node 30 and the donor eNodeB 20, referred to as “backhaul” communications, is performed using the Un interface on both the uplink (relay node to eNodeB) direction and the downlink (eNodeB to relay node) direction.
One of the main objects of micro/pico layers is to offload as many users as possible from the macro layers. In an ideal scenario, this may enable users to experience higher data rates in both the macro and micro/pico layers.
In this respect, several techniques have been discussed and proposed within 3GPP:
i) Extending the range of small cells by using cell specific cell selection offsets. A cell selection offset is an additional power margin for a cell that must be overcome before a handover to the cell will occur. Setting the cell selection offset for a particular microcell to a negative value can therefore increase the probability of a handover occurring to the microcell, thereby extending the range of the microcell.
ii) Increasing the transmission power of low power nodes and simultaneously setting appropriately the uplink (UL) power control target PO for the users connected to low power nodes.
The solution of deploying small base stations within the already existing macro layer grid is an appealing option, since these smaller base stations are anticipated to be more cost-efficient than macro base stations, and their deployment time is expected to be shorter as well. Even so, there will be scenarios in which deployment of pico- or macro-base stations and their associated backhaul costs may be prohibitive. In such scenarios, the use of relay nodes that employ in-band backhaul communications may provide a viable option that provides pico cell type coverage either indoors or outdoors and mitigates the cost and effort of deploying land-line backhaul to all the pico base stations.
One of the problems with heterogeneous networks employing relay nodes is that the RN backhaul link (Un) between the donor base station 20 and the RN 30 can generate additional interference above the levels that would normally be experienced, into the macro network, which may adversely affect the capacity of the macro network.
For example, Un uplink transmissions to a given macro donor eNodeB 20 from a RN 30 can cause interference into the backhaul Un uplink transmissions of relay nodes in adjacent macro donor cells. Furthermore, Un transmissions from an RN 30 within one macro cell could also interfere with uplink transmissions between the terminals or user equipment (UEs) to their serving RNs in neighboring cells. These interference scenarios are illustrated in FIG. 3.
FIG. 3 illustrates a portion of a wireless network including three macro cells 111a, 111b and 111c served respectively by macro base stations 20a, 20b, 20c. Each of the macro cells includes a plurality of relay nodes 30, including relay node 30a in macro cell 111a and relay node 30c in macro cell 111c. 
Each relay node defines an associated microcell, such as the microcell 120 defined by the relay node 30a in macro cell 111a, in which the relay node communicates with UEs 40 over Uu communication links. Moreover, each relay node communicates with its donor macro base station 20 over Un communication links to provide backhaul communications for the relay node. The Un communication links may be in-band links that use the same frequencies as are used in the microcell associated with the relay node. However, the Un and Uu communications may be orthogonal in time in order to reduce or avoid interference within the microcell 120. For example, the Un and Uu uplink communications at a relay node 40 may be time division multiplexed to occur at different times to reduce or avoid interference.
As further illustrated in FIG. 3, the primary lobe of a directional beam in the Un uplink formed by a relay node may be directed at the donor base station. For example, the primary lobe of the directional beam 131 generated by the relay node 30a is directed at the base station 20a. However, the side lobes and back lobe of the beam may be directed toward other cells. For example, the uplink Un beam formed by the relay node 30b in macro cell 111b may have a side/back lobe 141 that is directed at the relay node 30a and a side/back lobe 143 that is directed at a UR 40b in the macro cell 111a. These lobes may cause undesirable interference in Un communications in macro cell 111a. 
In addition, a side/back lobe directed at an eNB in a neighboring macro cell can cause interference to the Un uplink interface.
A reciprocal situation can occur in which downlink (DL) transmission on the Uu link between the relay node 30 and the UE 40 could cause interference into the downlink Un signals of neighboring cell RNs 30. These scenarios may be likely to occur, since it is anticipated that one of the likeliest deployment scenarios for relay nodes is that in which relay nodes are deployed at cell edges of neighbor donor macro base stations; hence it is very likely relay nodes might be close to each other.
In Long Term Evolution (LTE) systems, the existing approach to mitigating this interference involves employing time division multiplexing of the Un and Uu transmissions within a donor cell in order to help mitigate the potential Un to Uu interference, as described in TS 36.216, E-UTRA Physical Layer for Relaying Operation, v10.3. Thus, for example the Un and Uu transmissions within a picocell served by a relay node 30 may be separated in time.
This approach could also be employed in combination with directional antenna beams between the serving eNodeB and the given RN(s) as shown in FIG. 3.
There are two main issues with the existing approaches. First, the use of time division multiplexing of the Un and Uu transmissions can mitigate interference within a given donor macro cell, but it does not guarantee mitigation of interference between RNs of adjacent donor cells. Even though the RN may use directional antennas for Un link, the sidelobes and or backlobe of the RN antenna for the Un link can still cause significant interference to RNs (i.e., the Uu link) in neighboring macro cells, since these RNs could be in close proximity. Such a situation may occur when RNs are deployed near the cell edge of neighboring macro cells, which is the most likely deployment scenario for relay nodes, as mentioned above. It can be possible to restrict in the time domain when neighboring cell RNs can transmit on their respective Un and Uu links, however this would require strict time synchronization between neighboring macro cells and the RNs within the neighboring cells. In general cellular networks may not be synchronized, thus a solution for deployment of relay nodes in unsynchronized networks will also be of benefit.